This invention relates to a method of and a device for obtaining a non-integral delay, i.e. a delay of a signal by a time interval equal to a fraction of a clock period.
The non-prepublished European Patent Application no. 92.201.894.0, filed on Jun. 26, 1993, which corresponds to U.S. Pat. No. 5,349,548, and which together with the Applications corresponding thereto is herewith incorporated by reference, describes a Variable Phase Delay (VPD) FIR filter which can be used to shift a time-discrete signal in time. The phase delay can then be any arbitrary traction of a sampling period Ts. Such a variable delay is required, for example, to convert the signal to another sampling frequency Fs. The transfer function Fvpd of the VPD filter is:
Fvpd=F0+.delta.*[.delta.*F1+(1-.delta.)*sign*F2], where:
F0=low-pass-filtered (LPF) signal
F1=Sn-F0=first differential signal
Sn=nearest sample
F2=second differential signal
.delta.=shift relative to the centre of the sampling period, expressed with regard to 50% of a sampling period
sign=sign of the shift relative to Sn
FIGS. 1 and 2 show two embodiments, i.e. the VPD10 filter (with 10 delay sections Z.sup.-1) and the VPD04 filter (with 4delay sections), respectively, together with a resulting interpolation characteristic (gain G versus frequency in multiples of the sampling frequency Fs) for a delay by a .delta.=n/4 for n=-4..3 of a sampling period, which results in an interpolation yielding a signal sampled at 8*Fs. The timeshift .delta. is related to 50% of the sampling period because, starting from a minimum delay of half a sampling period, each delay can be realised by varying .delta. and, if necessary, adding a variable number of integer delay section. The shadow ranges in the interpolation characteristic are situated within bandwidths of 3/8*Fs around multiples of the sampling frequency Fs. The impulse response of the VPD10 filter, in the case of conversion to a substantially higher sampling frequency (small increment of delta), is shown in FIG. 3, together with the control signals sign(.delta.) and .delta..
If a VPD-filter is to be used for vertical interpolation of TV pictures (for example, in standard conversion for matrix display) the problem arises that a unit delay consists of a line memory which is substantially larger than the required adders and/or multipliers. This means that it is then preferable to opt for a short delay network (for example VPD04) and, if need be, to use more hardware for further processing. Moreover, a vertical prefilter would also require line memories so that it is preferred to have an interpolation filter having a "flatter" stop band and, consequently, slightly lower peaks around multiples of the transition band. Cf. around (N+0.5)*Fs in the characteristic shown in FIG. 2B.
For use in conjunction with audio signals and composite video signals, a smaller ripple would be desirable throughout the characteristic, particularly in the pass band and at harmonics thereof (around N*Fs, where Fs is the sampling frequency).
It is one of the objects of the invention to provide a non-integral delay circuit which meets at least some of the above-mentioned drawbacks and/or requirements. To this end, a first aspect of the invention provides a non-integral delay circuit as defined in Claim 1. A second aspect of the invention provides for a method as defined in Claim 7. Advantageous embodiments are defined in the dependent Claims.